Optical encoders are used in a wide variety of contexts to determine movement and/or a position of an object with respect to some reference. Optical encoding is often used in mechanical systems as an inexpensive and reliable way to measure and track motion among moving components. For instance, printers, scanners, photocopiers, fax machines, plotters, and other imaging systems often use optical encoding to track the movement of an image media, such as paper, as an image is printed on the media or an image is scanned from the media.
One common technique for motion encoding uses an optical encoder and an encoder pattern (or encoding media). The optical encoder focuses light on a surface of the encoder pattern. As the encoder pattern (or encoding media) moves with respect to the optical encoder, an optical sensor reads a pattern of light either transmitted through, or reflected by, the encoder pattern to detect the motion.
A typical encoder pattern is an alternating series of features. As the encoder pattern moves relative to the optical encoder (or vice versa), transitions from one feature to the next in the pattern are optically detected. For instance, an encoder pattern could be an alternating pattern of holes, or optically transmissive windows, in an opaque material. In that case, an optical sensor can detect transitions from darkness to light passing through the holes or windows.
FIG. 1 illustrates a basic motion encoder set 100 comprising: an optical encoder 110 including a light emitter 112 and an optical sensor 114; a housing 175 on which optical encoder 110 is mounted; a rotating shaft 150; and a code wheel 130 including an encoder pattern 132 disposed between the light emitter 112 and the optical sensor 114, mounted on the rotating shaft 150. Code wheel 130 rotates, thereby moving encoder pattern 132 relative to optical encoder 110.
In the embodiment of FIG. 1, optical encoder 110 operates in a transmissive mode by detecting light passed through encoder pattern 132 of code wheel 130. In another embodiment, light emitter 112 and optical sensor 114 could be disposed on the same side of code wheel 130 such that optical encoder 110 operates in a reflective mode by detecting light reflected by encoder pattern 132 of code wheel 130.
In one embodiment, encoder pattern 132 is an A/B pattern having alternating areas of differing optical transmissivity or reflectivity, depending on the design of optical encoder 110. Optical sensor 114 detects the rate of change between the A and B patterns and thereby ascertains the relative rotational movement between encoder pattern 132 and optical encoder 110.
However, due to wear and tear of code wheel 130 or shaft 150, or perhaps a bearing of shaft 150, the edge of code wheel 130 may eventually begin moving eccentrically (waggling and/or wobbling), and/or moving up and down within the encoder housing 175. If there is a waggling eccentricity in code wheel 150's motion, optical encoder 110 will not encode the rotational movement accurately, especially when the movement is less than one full revolution. Also, a wobbling code wheel 130, or an up/down movement of code wheel 130, may rub against or collide with housing 175, producing inaccurate motion detection signal(s) and possibly damaging housing 175 and/or code wheel 130.
In many cases, a motion encoder set is located internal to some host apparatus so that a waggling or wobbling code wheel, or a code wheel moving up and down in the encoder housing, is not easily observed and recognized. So, a user has no way of determining the magnitude of any waggling or up/down movement of the code wheel or shaft. Therefore, the user may not recognize that the motion encoder set is providing inaccurate signals which may impair operation of the host apparatus, or even damage the host apparatus, or that the motion encoder set itself can be damaged.
To address this problem, currently it is required that preventive maintenance be performed periodically on the optical encoder set to inspect for eccentric and/or up/down movement, and to make any necessary repairs and parts replacement. In many cases, this requires the host apparatus to be shut down and opened for inspection. As a result, this periodic maintenance is very expensive, and increases the down-time of the host apparatus. Furthermore, in many cases the maintenance is performed unnecessarily when there is no code wheel eccentricity or up/down movement, and the motion encoder set is performing perfectly.
What is needed, therefore is a motion encoder set that overcomes at least the shortcomings of known motion encoder sets.